block/vpc: Don't abuse the footer buffer as BAT sector buffer

create_dynamic_disk() takes a buffer holding the footer as first
argument.  It writes out the footer (512 bytes), then reuses the
buffer to initialize and write out the dynamic header (1024 bytes),
then reuses it again to initialize and write out BAT sectors (512).

Works, because the caller passes a buffer that is large enough for all
three purposes.  I hate that.

Use a separate buffer for writing out BAT sectors.  The next commit
will do the same for the dynamic header.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201217162003.1102738-3-armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/vpc.c

@@ -824,6 +824,7 @@ static int create_dynamic_disk(BlockBackend *blk, uint8_t *buf,
 {
     VHDDynDiskHeader *dyndisk_header =
         (VHDDynDiskHeader *) buf;
+    uint8_t bat_sector[512];
     size_t block_size, num_bat_entries;
     int i;
     int ret;
@@ -847,9 +848,9 @@ static int create_dynamic_disk(BlockBackend *blk, uint8_t *buf,
     /* Write the initial BAT */
     offset = 3 * 512;
 
-    memset(buf, 0xFF, 512);
+    memset(bat_sector, 0xFF, 512);
     for (i = 0; i < DIV_ROUND_UP(num_bat_entries * 4, 512); i++) {
-        ret = blk_pwrite(blk, offset, buf, 512, 0);
+        ret = blk_pwrite(blk, offset, bat_sector, 512, 0);
         if (ret < 0) {
             goto fail;
         }